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The momentum anisotropy contained in a sheared flow may be transferred to a pressure anisotropy, both gyrotropic and non-gyrotropic, via the action of the fluid strain on the pressure tensor components. In particular, it is the traceless symmetric part of the strain tensor (i.e. the so-called shear tensor) that drives the mechanism, the fluid vorticity just inducing rotations of the pressure tensor components. This possible mechanism of anisotropy generation from an initially isotropic pressure is purely dynamical and can be described in a fluid framework where the full pressure tensor evolution is retained. Here, we interpret the correlation between vorticity and anisotropy, often observed in numerical simulations of solar wind turbulence, as due to the correlation between shear rate tensor and fluid vorticity. We then discuss some implications of this analysis for the onset of the Kelvin–Helmholtz instability in collisionless plasmas where a full pressure tensor evolution is allowed, and for the modelling of secondary reconnection in turbulence.

Shear-induced pressure anisotropization and correlation with fluid vorticity in a low collisionality plasma